How to mask normal maps or How to select and apply portions of normal map?
In comparison to bump maps were black and white are 0% and 100% influence, normal maps work from a blueish base tone.

rgb (128,128,256): is 0% normal map influence


you can use your bw-mask as a factorial in the color-mix-node while using your normal map as one input and the rgb color(128,128,256) as a second input.